A 'square rig' for seamen was introduced in 1857, with badges worn on the left sleeve to denote differences between senior and junior ratings. Petty Officers moved into 'fore' and 'aft' rig of jacket, waistcoat, trousers and peaked caps in 1859.
Me thinks you refer to the Corpral/Sergeant stripes worn by Royal Marines. These indicate the same ranks as the army with a variantion in name and are only worn on the left arm by Marines as opposed to both by the army. Having recently attended a passing out cerermony of 6 Army Staff Sergeant's and 1 Royal Marine Colour Sergeant I also questioned this fact.
Sailors wear one, two or three stripes on their right arm under any badge of rank to denote 4 years of undetected crime up to three stripes or 12 years.